Carrier and speech transmission system



March 21, 1939. H. wHlTTLE CARRIER AND SPEECH TRANSMISSION SYSTEM FiledOct. 6, 1936 nu .um mw um ,7.l F F P N.' P H L FI m/T m F. F MII P. m1]P. u L. nu IA. Ml. mm Nimm mu uw rm. c rl. T

sua's s51- 20 -I F/L 15k E doo 25 sussen/@ERS SET I [mw T l T D" l T 1rb L//vE/o 5 FREQUENCY /Nf/ENTOR l. WH/TTLE y Patented Mar. 21, 1939UNITED STATES CARRIER AND TRANSIVUSSION TEM Horace Whittle, Maplewood,N. J., assignor to Bell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New Yori; Application October 6, 1936,Serial No. 1041,183

2 Claims.

The present invention relates to carrier telephone systems on lines thatare also used for ordinary telephony and in particular to the provisionof lters in the subscribers lines for preventing the carrier frequencycurrents from entering the voice frequency apparatus.

In some types of lines, such, for example, as train despatching lines,it is customary to have a number of subscriber stations adapted forconnection in bridge of the line. If a carrier telephone system i s tobe superimposed on the same line it is necessary to provide selectivecircuits in the sub-set branches to prevent undue shunting of the highfrequency currents used for car- 13 rier communication. If sufficientlyhigh frequencies were used for the carrier communication, theseselective circuits could be of quite simple construction and mightreduce to a simple inductance or resonant circuit. It is desirable,however, to keep the frequencies used for carrier transmission to as lowa frequency range as practicable because of such factors as cross-talk,attenuation and variations in transmission characteristics with Weatherconditions on open-wire lines, since all of these effects increase asthe frequency increases.

Where the frequencies used for carrier transmission are brought down solow that a rather carefully designed band filter is required for thebridged sub-sets, a diicult problem is encountered because of the factthat the usual bridging sub-set is designed to have Widely differentimpedances for the transmitting and receiving conditions.

This problem is solved according to the invention by providing in thepreferred form of embodiment, two filters for the talking and listeningconditions so designed as to provide optimum conditions of transmissionfor the voice currents with adequate discrimination against thefrequencies used for carrier transmission.

The objects of the invention and its mode of application in practicewill be made clear in the following description with the aid of theaccompanying drawing, in which Fig. 1 is a block schematic circuitdiagram of a system to which the invention is particularly applicable;

Fig. 2 is a schematic diagram of a substation circuit equipped withfilters in accordance with the invention; and

Fig. 3 shows insertion and bridging loss characteristic curves for a.system incorporating the invention.

55 In Fig. 1 the telephone line or train despatching line l0 has a voiceterminal at Il which may be a despatchers telephone set of standard typefor communicating over the line with bridged substations, two of whichare shown at l2 and i3. There may be a large number of these 5 bridgedstations, in accordance with usual practice. Carrierterminals areprovided at I4 and I5V for communicating over line l. A high-pass filterI6 or l1 is connected between the respective carrier terminal and lineand a low-pass filter lo I8 or i9 is placed in the voice current branchof the line as in usual practice.

A bridging filter 2li or 2l is shown for connection between line lll andrespective substation set I2 or i3, these filters being for the purpose15 of preventing the shuntlng off by the bridged circuits of thecurrents used in carrier communication on line l0. In Fig. 1 the filters20 and 2| are merely indicated as included in the circuits in bridge ofthe line. In case of sufficiently great 90 frequency separation betweencurrents used for the carrier and voice communication channels, a simplefrequency selective circuit such as a tuned circuit or possibly a seriesinductance might suffice at the position indicated by 20 or 2|- 25 Inone system to which the invention has been applied, however, the carrierfrequency was 7150 cycles and the lower side-band (used in West to easttransmission) extended downward in frequency to about 4150 cycleswhereas the speech 30 channel utilized frequencies up to about 2750cycles. These values of frequency in relation to the other factors thatwere involved required a special system of filters for the substationcircuit branches, as will now be described in con- 35 nection with Fig.2.

The substation circuits used on line l0 comprised, before the inclusionof the carrier discriminating filters, a condenser 25, a repeating coil26 equipped with a talk-listen key 21 ar- A( ranged to include the wholecoil in circuit in the 4 listening condition and a small part of thecoil in the talking condition; a receiver 28 and a transmitter 29. 'Iheresistance of the receiver was of the order of ten times that of thetrans- 45 mitter. A talking current battery 30 was arranged to beconnected into and out of the circuit by a contact on the key 21. Theswitchhook is shown at 3|.

With the entire coll 26 in circuit (key 2l re- 50 leased) the substationset presented a. high impedance across the line. (The line to dropimpedance ratio of this coil may, for example, be to 1.) A large numberof sets could, therefore, be bridged on the line in the listeningcondition without producing too great a bridging loss to speech currentstransmitted over the line from stationV il or from any bridgedsubstation. It was necessary in talking to depress key 21 to remove allbut a small part of the impedance of coil 26 from the circuit in orderto give the proper mutual impedance and low direct current resistance tothel battery current to secure eificlent transmission from thetransmitter 29 out on to the line. Battery 30 in the talking position ofkey 21 is connected through only the lower part of coll 26 which has alow direct current resistance. v

Superposing the carrier communication channel on the line i0 raised theproblem of providing in the substation branches suillcient'discrimination against the carrlercommunication currents to prevent toogreat a bridging loss in the carrier frequency range regardless of thenumber of substation circuits in use at one time, and also of keepingtheattenuation and impedance at such values in the voice frequency rangeas not to impair speech Vtransmission over the system.

'Ihis was accomplished by providing separate filters having quitedifferent nominal impedance but similar cut-off characteristics in thetalking and receiving branches of the circuit, shown respectively at 33and 34. These are low-pass illters with a suppression peak in theneighborhood of 5 kilocycles, a typical characteristic being given bycurve A in Fig. 3.

With key 21 released, and therefore with filter 34 in circuit and filter33 out of the circuit, lter 34 is terminated on the line side by theimpedance of the line transformed byA coil 26, the whole of which is incircuit. It is terminated on its opposite end in the receiver 23 and anG-ohm resistance 36. This resistance is for the purpose of providing aterminating resistance to filter 34 when the receiver is on theswitchhook 3|. It performs no useful function when the receiver is incircuit Vbut the loss due to it is not serious.

With key 21 depressed, lter 33 is brought into circuit and is terminatedon one end by transmitter 29 and on the other end by the line impedancetransformed through the lower portion of coil 26. In this condition ofthe circuit lter 34 is also in circuit, fllter 34 being connected acrossthe lower part of the drop side of coil 26 in parallel with the outputterminals of -iilter 33.

It is to be noted that a number of the bridged sets may be listeningacross line i0 at the same time but that in general only one will betalking at a time. It is necessary therefore to provide high impedanceas well as attenuation for the carrier frequencies in the receivingcondition of the substation circuit. It is not so necessary to providesuch high impedance in the talking con-- dition.

The bridging loss curve for one substation circuit of Fig. 2 in itsreceiving or in its idle condition is given by curve B in Fig. 3. Thepeak in the loss curve which occurs unavoidably due to the presence ofthe filter critical or cut-off frequency, falls in between the twoutilized frequency ranges, namely, the voice and carrier ranges. Theloss is seen to be of the order of only .01 o r .02 decibel over thecarrier range. and less than- 0.1 decibel in the voice range. In

the transmitting condition the los in the car-l east. The upperside-band extended up to 10,150

cycles.

The circuits and .'specic example of their use that have been given areto be regarded as illustrative rather than limiting. The scope of theinvention is indicated in the claims.

What is claimed is: f V

1. In combination with a line carrying currents in the voice frequencyrange. and carrier communication currents of frequencies slightly higherthan the upper frequency of said currents of the voice frequency range,a substation circuit including a transmitter and a receiver of differingimpedances, and a repeating coil, said circuit in listening conditionincluding said coil, said receiver and a filter across the line, thetransmitter being disconnected from the line, and in talking conditionincluding a portion of said coil, said transmitter and a second filteracross the line, said repeating coil being connectedl between said lineand said filters and said filters freely transmitting the voicefrequency currents between the line and receiver and transmitter whileoffering high attenuation to currents of the frequencies used forcarrier communication. Y

2 In combination with a line 'carrying currents in the voice frequencyrange and carrier communication currents of frequencies higher than theupper frequency of said currents of the voice frequency range, the lowerfrequency of the carrier communication currents being close to the upperfrequency of the currents in the .volcerange,'a substation circuitincluding a receiver and a transmitter of differing impedances, arepeating coil having a primary winding and a secondary winding, saidprimary winding being connected across said line, said circuit inlistening condition having the receiver, a filter and the secondarywinding connected together, and in talking condition having thetransmitter, a portion of said secondary winding and a second filterconnected together, said filters freely transmitting voice currentsbetween the line and receiver and transmitter while offering highattenuation to currents of the frequencies used for carriercommunication.

. HORACE WHI'I'I'LE.

